It is well known in the art that aromatic carboxylic acids, as the potassium salts, can be isomerized or disproportionated via the Henkel reaction to form aromatic para substituted diacids. In this way, phthalic acid salts or benzoic salts, for example, can be made into terephthalic acid salts (TPA salts) and naphthoic acid salts or other isomers of naphthalene dicarboxylic acid (NDA) may be made into 2,6-naphthalene dicarboxylic acid (2,6-NDA). These compounds are often used as monomers for the preparation of polymeric materials. 2,6-naphthalene dicarboxylic acid (2,6-NDA) is a particularly useful aromatic carboxylic acid, because it can be reacted with ethylene glycol to prepare poly(ethylene-2,6-naphthalate), PEN.
The reaction of phenol with base to form salicylic acid at ca. 150-200.degree. C. with a Na.sup.+ counter ion by the addition of CO.sub.2, or parahydroxy benzoic acid at high temperature(225-300.degree. C. with K.sup.+ counter ion) is known in the literature, but it has never been observed that terephthalic acid (TPA) could be produced by either reaction. Neither has anything been found in the art to suggest that phenols could be useful to the Henkel reaction.
There is nothing found in the art to suggest that terephthalic acid could be produced directly from phenol, or that naphthalene dicarboxylic acid could be produced directly from naphthol. It would be of great value in the art if reactions such as this were possible, because phenol and naphthol are quite inexpensive as raw materials. Phenol can be prepared extremely inexpensively, at about one-third the cost of benzoic acid. Likewise, naphthol is readily available from naphthalene, for example by sulfonation or sulfation followed by hydrolysis, without the use of the oxidation reactions needed to form naphthalene dicarboxylic acid or naphthoic acid from hydrocarbons.
In addition, it would be very advantageous if a reaction to produce valuable diacids could be carried out without an oxidation step. A process that does not require oxidation could obviate the necessity of air separation plants, thus lessening the required scale for economic feasibility. It would be very desirable if there were a safe, oxidation-free route from common precursors, such as phenolic compounds, to desired diacid monomers for polyesters.
In the present invention, we have discovered a novel reaction comprising the conversion of phenolic compounds to diacids by the action of high temperature and base. Since the diacid products are of exceptionally high quality, the process is therefore a convenient way to make a high value product from a low cost raw material by a non-oxidative route.